Bagrut Chemistry key terms

A relative measure of how strongly an atom in a bond attracts the shared electron pair. It increases across a period and decreases down a group, and large differences between bonded atoms produce polar or ionic bonds.

The minimum energy needed to remove one electron from an isolated gaseous atom or ion. Successive values increase, and a large jump reveals how many valence electrons the atom has.

A measure of the size of an atom, usually half the distance between nuclei of two bonded atoms or of two touching atoms. It decreases across a period and increases down a group.

The net positive charge from the nucleus that a particular electron actually feels after inner electrons screen it. It grows across a period and helps explain shrinking radii and rising ionization energies.

The electrons in the outermost occupied shell of an atom that take part in bonding and determine chemical behavior. Main-group elements show a number equal to their group number for groups 1-2 and 13-18.

Four integers (n, l, ml, ms) that completely describe the state of one electron in an atom. No two electrons in the same atom share all four, which is the Pauli exclusion principle.

A region of space around a nucleus where the probability of finding a given electron is high, holding at most two electrons of opposite spin. Its shape is labeled s, p, d or f.

When electrons fill orbitals of equal energy, they occupy separate orbitals with parallel spins before any pairing occurs. This minimizes electron-electron repulsion and explains unpaired electrons in atoms.

The tendency of atoms to gain, lose or share electrons so that each ends with eight electrons in its outer shell. It predicts formulas for many main-group compounds but has exceptions such as expanded octets.

A diagram showing atoms, bonding electron pairs as lines and lone pairs as dots, used to predict molecular shape and bond polarity. Formal charge and octet counting help check whether the structure is reasonable.

A model that predicts molecular geometry from the number of electron domains around a central atom. Domains repel and spread out, and lone pairs occupy more space than bonding pairs, compressing bond angles.

A bond in which shared electrons are pulled toward the more electronegative atom, creating partial charges. The bond lies between purely covalent and fully ionic behavior on the electronegativity scale.

The net dipole of a whole molecule, obtained by adding individual bond dipoles as vectors. Symmetric geometry can cancel polar bonds, so a molecule may be nonpolar despite having polar bonds.

An unusually strong attraction between a hydrogen atom bonded to N, O or F and a lone pair on a nearby electronegative atom. It raises boiling points and shapes proteins and DNA.

Attractions between separate molecules or ions, much weaker than covalent bonds, that govern boiling points, solubility and phase behavior. They include dispersion, dipole-dipole and hydrogen-bonding interactions.

Weak, temporary attractions caused by instantaneous fluctuations of electron density in any atom or molecule. They strengthen with more electrons and greater surface contact, so larger molecules generally have higher boiling points.

How easily an electron cloud can be distorted by an external charge or nearby molecule. Larger, more diffuse clouds are more polarizable, which strengthens dispersion forces and often raises boiling points.

The amount of substance containing exactly 6.022 x 10^23 elementary entities, which is Avogadro's number. It links countable particles to measurable mass through molar mass.

The mass of one mole of a substance in grams per mole, numerically equal to its formula mass in atomic mass units. It converts between mass and amount of substance in stoichiometric calculations.

The reactant that is completely consumed first and therefore fixes the maximum amount of product formed. Comparing mole ratios, not masses, identifies it and determines theoretical yield.

The maximum mass or amount of product predicted from stoichiometry when the limiting reactant is fully used. Dividing the actual yield by it and multiplying by 100 gives percent yield.

The simplest whole-number ratio of atoms of each element in a compound. It is found by converting percentage composition to moles and dividing by the smallest value.

The actual number of atoms of each element in one molecule of a compound. It equals the empirical formula multiplied by an integer found from the ratio of molar mass to empirical formula mass.

The equation PV = nRT relating pressure, volume, amount of gas and absolute temperature, with R the gas constant. It assumes negligible particle volume and no intermolecular attractions.

The pressure a single gas in a mixture would exert if it alone occupied the same volume at the same temperature. Dalton's law states the total pressure is the sum of partial pressures.

The number of moles of one component divided by the total number of moles in a mixture. It equals the ratio of that component's partial pressure to total pressure in an ideal gas mixture.

The volume occupied by one mole of an ideal gas at a defined reference condition, about 22.4 liters at 0 degrees Celsius and 1 atmosphere. It allows converting gas volume to moles.

A graph of pressure against temperature showing which phase of a substance is stable and marking the triple point and critical point. Lines indicate conditions where two phases coexist in equilibrium.

The unique temperature and pressure at which a substance's solid, liquid and gas phases coexist in equilibrium. On a phase diagram it is where the three coexistence curves meet.

The temperature and pressure above which a liquid and its vapor become indistinguishable, forming a supercritical fluid. Beyond it, no amount of pressure can liquefy the gas.

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, it shifts in the direction that partly opposes the change. Catalysts do not shift the position of equilibrium.

The ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium for a given temperature. Its size indicates whether products or reactants dominate.

The same ratio as the equilibrium constant but calculated with current, not necessarily equilibrium, concentrations. Comparing Q with K predicts whether the reaction proceeds forward or backward.

The negative base-10 logarithm of the hydrogen ion concentration in moles per liter. It indicates acidity on a scale where 7 is neutral at 25 degrees Celsius.

An acid that only partly dissociates in water, establishing an equilibrium described by an acid dissociation constant, Ka. Its conjugate base is relatively strong and solutions show limited conductivity.

A solution that resists large pH changes when small amounts of strong acid or base are added, usually made from a weak acid and its conjugate base. It works best near the acid's pKa.

The heat exchanged with the surroundings at constant pressure during a chemical reaction, given per mole of reaction. A negative value means the reaction releases heat and is exothermic.

The total enthalpy change of a reaction is the same regardless of the route taken, because enthalpy is a state function. It allows summing known enthalpy changes to find an unknown one.

The average energy required to break one mole of a particular covalent bond in the gas phase. Summing broken minus formed bond energies estimates the enthalpy change of a gas-phase reaction.

The potential of a half-cell measured against the standard hydrogen electrode, which is assigned zero volts. More positive values indicate a greater tendency to be reduced.

An electrochemical device that converts the energy of a spontaneous redox reaction into electrical work. Oxidation occurs at the anode and reduction at the cathode, connected by a salt bridge.

The species that accepts electrons and is itself reduced, causing another substance to be oxidized. Its strength increases with a more positive standard reduction potential.

The minimum energy that colliding particles must possess for a reaction to occur, corresponding to the top of the energy barrier. A catalyst lowers it without changing the overall enthalpy change.

A substance that speeds up a reaction by providing an alternative pathway with lower activation energy while being regenerated. It does not alter the equilibrium position or the enthalpy change.

The slowest elementary step in a multistep mechanism, which limits the overall reaction rate. Its composition determines the observed rate law for the process.

A specific group of atoms within a molecule that gives it characteristic chemical properties and reactivity, such as hydroxyl, carboxyl or carbonyl groups. It determines the compound's class and typical reactions.

The existence of compounds with the same molecular formula but different arrangements of atoms, giving different properties. Structural isomers differ in connectivity, while stereoisomers differ only in spatial arrangement.

The property of a molecule that cannot be superimposed on its mirror image, usually due to a carbon bearing four different groups. Such enantiomers rotate plane-polarized light in opposite directions.

A family of organic compounds with the same functional group and general formula, where successive members differ by one CH2 unit. Members show gradually changing physical properties and similar chemical behavior.